Traditionally, there are several ways to provide high quality graphics in gaming machines. These include:                Images which are stored in EPROM or hard disk and which are played back on the screen using traditional 2D techniques. Animations are commonly used to improve the presentation and some products store large amounts of animation on hard disk drive. Animations may be generated from 3D modelling software and pre-rendered into a 2D animation; or        Real-time 3D which is used to generate images from 3D models stored in memory.        
In theory, the same scenes can be generated by either method. Pre-rendered graphics are produced ahead of time and played back like a video. The 3D system recreates the image in real-time. The 2D system has the potential for superior image quality at the cost of flexibility. The real-time 3D system has flexibility, but poorer image quality.
There are many effects that are difficult or impossible to pre-render into 2D, yet are easy to perform in real-time 3D. On the other hand there are many effects that are easy to pre-render, yet very hard or impossible to perform with real-time 3D.
An example of the flexibility of 3D is the rippling water effect, in which, after a win, the entire screen is rippled as if it were water. This is very simple to achieve in 3D, as the screen is simply divided into a large number of polygons which are moved in a wave. It does not matter what image is overlaid on the polygon mesh. The same effect is effectively impossible to pre-render because of the large number of possible stopping positions on the reel strips (perhaps 100 million).
An example of the quality of 2D is a photograph or video of, say, a person. It is not possible to model a person convincingly in real-time 3D, yet it is very simple to take a photograph or video.